The overall objective of the University of Chicago Population Research Center is to provide an environment in which multidisciplinary research on basic scientific and clinical aspects of the population problem can be facilitated and encouraged. The Center is already in its fourth year of operation. It is hoped that it can continue to flourish along the following lines. The Center supports eight major core facilities for (1) ultrastructural analysis, (2) protein analysis, (3) cytogenetics, (4) histopathology, (5) endocrine assay, (6) gas chromatography-mass spectrometry, (7) specimen storage, and (8) analytical ultracentrifugation. These core facilities are overseen by outstanding experts in each area. They provide not only equipment and technical help for the appropriate measurements, but also expert advice in scientific interpretations. Additional core facilities may be added if the needs arise. The Center currently involves 29 established investigators with primary appointments in eight Departments of the Division of Biological Sciences, some of whom hold joint appointments in three other Departments. The Center also supports some features of animal experimentation, and a seminar and lecture series. It additionally provides counsel by Center members to students and faculty interested in research in productive biology. The research projects and collaborative programs, including laboratory and clinical investigations, deal with such diverse topics as biological and biochemical aspects of ovarian, testicular and placental fudction; the actions of sex hormones, prostaglandins, and their antagonists; cytogenetics; chemical physiology of male and female secondary sexual tissues; aspects of reproductive behavior; meiosis; infertility in the human male and female; the physiology of spermatozoa and of male and female genital secretions; and the effects of reproductive hormones on renal and cardiovascular functions.